Protocol and system description
The X10 protocol consists of bit "address" and then bit "command". For example, you can say "lamp #3", "turn on!" - and it happens. You can address several units before giving a command: "lamp #3", "lamp#12", "turn on!".
Appliances are generally plugged into X10 modules (receivers). X10 makes a distinction between "lamp modules" and "appliance modules". Appliance modules supply electrical devices with power and accept X-10 commands. Appliance modules are capable of handling larger loads (i.e. coffee maker machine, heaters, motors,...), simply turning them on and off.
Therefore, if you wished to control a light via X-10 commands, you would plug the light into a light module and the module into the mains. You would assign it an address (A1, for example). Now, when you sent the "A1 on" command down the power lines, the light would turn on. Lamp modules aren't capable of running heavy loads.
Each device receiver is set to a certain unit ID (X10 addresses), and reacts only to commands addressed to it. Receivers ignore commands not addressed to them. X10 allows up to 256 distinct devices (addresses) on a home power system: 16 unit codes (module numbers) ,1- 16, for each of 16 house codes, A-P, (16 X 16 = 256). If 16 unit codes is not enough, even with sharing unit codes, you might consider using more than one house code.
The "base stations" (transmitters) was a simple control box that could be programmed to transmit various X-10 commands down the power lines. Modern methods consist of either using a RF remote control (that interfaces with a radio receiver) or via your computer using special software.
All commands are broadcast through the power mains therefore requiring no additional wiring. There are several commands, the basic ones being: on, off, , all on, all off, bright and dim.
In the 60 Hz AC power flow, a Binary Digit (bit) 1 is represented by a 1 millisecond burst of 120 kHz at the zero crossing point (0ยบ, but certainly within 200 microseconds of the zero crossing point), immediately followed by the absence of a pulse. And a Binary 0 by the absence of 120 kHz at the zero crossing points (pulse), immediately followed by the presence of a pulse. All messages are sent twice to reduce false signaling. After allowing for retransmission, line control, etc, data rates are around 20 bit/s. Obviously, X10 data transmission is so slow that the technology is confined to turning devices on and off or other very simple operations.
In order to provide a predictable start point, every data frame would always begin with a start code of "pulse", "pulse", "pulse", "absence of a pulse" (or 1110). Immediately after the start code, a letter code (A-P) is sent and after the code, comes a function code( number code,1-16, or command code, the selection of which is determinated by the last bit, 0=address number and 1=command). One start code, one letter code and one function code is know like a frame.
Each signal is also sent two times to make sure the receivers understand it over the "noise" of the power lines (for purposes of redundancy, reliability and to accommodate line repeaters).
Whenever the data changes from one address to another address, from an address to a command, or from one command to another command, the data frames must be separated by at least 6 clear zero crossings (or "000000"). The sequence of six "zero's" resets the shift registers.
Weak points and limitations
One problem with X10 is excessive attenuation of signals between the two out of phase 110 volt lines used in much North American construction. This may come and go when 230 volt devices like stoves or dryers that bridge the two circuits are turned on or off. In Europe one uses 230 V devices.
Other problems: TVs or wireless devices may cause spurious off or on signals. Noise filtering may help keep external noise out of X10 signals, but noise filters not designed for X10 may filter out X10 signals.
Some X10 controllers may not work well or at all with low power devices (below 50 watts) or devices like fluorescent bulbs that do not present resistive loads.
X10 signals can only be transmitted one command at a time. If two X10 signals are transmitted at the same time, they will collide and the receivers will not be able to decode the signal commands.
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